
.outer {
    margin-left: auto;
    margin-right: auto;
    background-color: #dddddd;
    overflow: hidden;
}
.img,.outer{
    width:900px;
    height: 500px;
}
#img1{
    background-image: url("../img/cat.jpg");
}
#img2{
    background-image: url("../img/flower.jpg");
}
#img3{
    background-image: url("../img/flower1.jpg");
}
#img4{
    background-image: url("../img/top.jpg");
}
.img{
    float:left;
}
#banner{
    transition: margin-left 1s;
}